1. Field of the Invention
This invention relates generally to the fabrication of integrated circuit devices and, more particularly, to the formation of circuit contacts in dynamic random access memory (DRAM) integrated circuit devices.
2. Description of the Related Art
The development of dynamic random access memory devices has now progressed through several generations. Each succeeding generation of DRAM devices has provided more storage capacity and has provided integrated circuit components which have smaller dimensions than the prior generations. This continuing development of the DRAM technology is generally referred to as higher scale integration. The increased scale integration has necessitated a variety of design changes. One of the changes has been the introduction of a "capacitor over bit line" (COB) memory cell structure. The COB memory cell structure replaced the "capacitor under bit line" (CUB) structure which uses aluminum and tungsten in the fabrication of the bit lines. Processes which are performed after the fabrication of the bit lines are characterized as low temperature processes, processes having a maximum temperature of approximately 450.degree. C.
Designs of DRAM units which include the COB structure avoid the use of metal bit lines because of metal bit line oxidation processes in the temperature range of 800.degree. C.-900.degree. C. This temperature range is used in the fabrication of capacitors, gate oxides and interlayer insulators. Consequently, the bit lines in the COB devices are fabricated using poly-silicon (poly-Si) and poly-side (WSi or TiSi.sub.x) materials. Although the poly-silicon and the poly-side materials can be fabricated to provide acceptable bit lines within the memory cell array, these materials cannot be used to make contacts with the peripheral circuits because the peripheral circuits are fabricated in complementary metal oxide semiconductor (CMOS) technology. Thus, some devices are connected to p-type regions in the substrate and some devices are connected to n-type regions in the substrate. The poly-silicon materials and the poly-side materials are doped in order to provide proper conductivity. The doping results in diode structure between the contacts and doped regions having an opposite polarity in the substrate. This diode structure cannot provide the functionality of the ohmic contact required for the connection between the bit lines and doped regions.
A need has therefore been felt for materials capable of making an ohmic contact with the metal which avoids the oxidation problem and which does not require impurity doping to achieve good conductivity.